This invention relates to radio frequency (RF) transmitting systems. More particularly, this invention is directed to the improvement in RF transmitting circuits which comprises a full-wave or bridge rectifier connected in series between (a) an RF signal generator coupled in parallel with an audio signal generator and (b) an antenna, whereby the full-wave rectifier serves to modulate the output of the RF signal generator with the output of the audio signal generator to produce a modulated, full-wave rectified signal to the antenna.
Rectifiers are well known in the art. For example, they are utilized in power supply systems for supplying direct-current (d-c) voltage, in RF detectors, etc. Some prior art examples of rectifier circuits and their uses are found in The Radio Amateur's Handbook--The Standard Manual of Amateur Radio Communication, published by the American Radio Relay League, Fifty-Fourth Edition, 1977, incorporated by reference herein.
RF transmitting systems are also well known in the art. The basic principles of RF transmission are set out in Chapter 6 of The Radio Amateur's Handbook referred to above.
Communication of messages by RF transmission involves the process of modulation. The broad definition of modulation implies three fundamental concepts: modulating wave, carrier and modulated wave. A modulating wave changes some parameter of the wave to be modulated; a carrier is a wave suitable for modulation by the modulating wave; and, lastly, a modulated wave is a wave, some parameter of which is changed in accordance with the modulating wave. For example, conventional amplitude modulation (AM) is defined as modulation in which the amplitude factor of a sine-wave carrier is linearly proportional to the amplitude of the modulating wave. Analysis shows that an amplitude modulated wave is composed of the carrier, which conveys no information apart from its own particular amplitude, frequency, and phase, plus the familiar upper and lower sidebands, which convey identical and therefore mutually redundant information. An important characteristic of AM transmission is that, apart from a scale factor and constant term, either the upper or lower envelope of the modulated signal is an exact replica of the modulating signal, provided two conditions are satisfied: first, that the carrier frequency exceeds twice the highest modulating signal frequency to be transmitted; and, second, that the carrier is not overmodulated. AM transmission has the advantages that the wave form of the message is preserved and the receiver circuit is relatively simple. However, AM transmission doubles bandwidth occupancy and also requires extra signal power, not only because two sidebands are transmitted, but also because the carrier is transmitted.
Single-sideband, suppressed carrier (SSB) modulation results from elimination of one sideband and reduction or elimination of the carrier of an AM signal. Assuming adequate knowledge of the carrier, the sideband which remains unambiguously defines the message.
Since one of the sidebands and possibly the carrier are eliminated, SSB transmission saves bandwidth occupancy and conserves signal power compared with AM transmission. However, SSB transmission cannot handle low frequencies. Additionally, an inherent delay results in SSB transmission as a consequence of elimination of the unwanted sideband. Furthermore, unlike in the case of AM transmission, the wave form of the wanted message is not preserved, and therefore an SSB receiver circuit is more complex than an AM receiver circuit. Moreover, if the carrier is eliminated prior to transmission, the SSB receiver must include an RF signal generator having the precise frequency and phase of the carrier in order to detect the message.
Angle modulation is defined as modulation in which the angle (entire argument) of a sine-wave carrier is the parameter changed by the modulating wave. Frequency and phase modulation are particular forms of angle modulation. Frequency modulation (FM) is angle modulation in which the instantaneous frequency of the sine-wave carrier is caused to depart from the carrier frequency by an amount proportional to the instantaneous magnitude of the modulating wave. Phase modulation (PM) is angle modulation in which the angle of a sine-wave carrier is caused to depart from the carrier angle by an amount proportional to the instantaneous magnitude of the modulating wave; that is, the linearly increasing angle of the sine-wave carrier has added to it a phase angle proportional to the instantaneous magnitude of the modulating wave. FM and PM are similar in the sense that any attempt to shift frequency or phase is accompanied by a change in the other.
In comparison with AM and SSB transmission, angle modulation transmission reduces noise in exchange for extra bandwidth occupancy and is characterized by constant average signal power and constant peak power that is only twice the average power. Also, angle modulation transmission exhibits a channel-grabbing property whereby if two signals reach the receiver circuit, the larger signal is detected to the near exclusion of the smaller. However, since angle modulation transmission is extravagant of bandwidth occupancy, signal power must be adequate to override wide-band noise. Also, a continuous wave (CW) signal at the carrier frequency is transmitted in the absence of a modulating signal. Furthermore, FM and PM receiver circuits include circuitry for converting frequency or changes in phase, respectively, into amplitude variations and are therefore more complicated than AM or SSB receiver circuits.